(a) Field of the Invention
The present invention relates to a prestressed composite truss girder and construction method of the same. More particularly, it relates to a prestressed composite truss girder made by combining lower-chord member composed of prestressed concrete structure with web member composed of rolled steel and upper-chord member composed of structural steel plate, and to construction method of the same.
(b) Description of the Related Art
Generally, the composite girders are composed of precast beam which is manufactured beforehand at a factory or a manufactory and slab concrete combined with said beam, and the bending stress and the shear stress occur when they are subjected to the external loads. In such composite girders, concrete that has strong resistance against compression is used for slab corresponding to the compression region, and steel or prestressed concrete that are highly resistant against tensile and shear stress is used for the precast beam which are mainly subjected to tensile and shear stresses.
Thus, composite girders which are applied at varieties of architectures and engineering structures could be classified into 4 sorts, i.e., steel composite girder, steel reinforced concrete(SRC) composite girder, preflex composite girder and prestressed concrete(PSC) composite girder according to the used material and manufacturing method. Among them, the steel composite girder and the SRC composite girder are non-prestressed structure wherein prestress is not introduced to the cross-section of the girder, and the Preflex composite girder and the PSC composite girder are prestressed structure wherein prestress is introduced when preparing the beam. And, these 4 sorts of composite girders have common point in that all the cross-sectional shape of the beam are solid web style.
As shown at FIG. 1, the steel composite girder (10) consists of I-shape steel to resist the bending stress and shear stress generated by dead load of the steel beam and slab concrete before composition, and the tensile stress caused by external loads after composition. The steel composite girder has advantages in that it could be easily constructed because of its light structure, it could have excellent resistance against earthquake, it could have sufficient ductility against destruction, and the period for site-work could be reduced to somewhat extent.
However, the steel composite girder also has disadvantages in that the material cost is high, noise and vibration by moving loads are heavy, and maintenance and repairing cost take so much. Further, because the steel composite girder has relatively weak stiffness, the height of the beam should be sharply increased to satisfy the deflection limitation against dynamic load when the span length exceeds about 40 m on the basis of simply supported structure system. From this reason, the spaces under the girder often cause problems, and the economical efficiency is remarkably lowered because the amount of steel used increases extremely. Moreover, when the composite girder has continuous span, a negative moment occurs at middle support region by the external load. In this case, the tensile stress occurs at slab concrete which is weak in tension and the compressive stress occurs at the steel girder which is weak in compression. As a result, it causes extraordinary increase of the construction cost compared to the simply supported structure, and the serviceability and the durability of the composite girder will be deteriorated by water leakage caused from the crack of the slab concrete.
As shown at FIG. 2, the SRC composite girder (20) is composed of H-beam and encasing reinforced concrete. The SRC girder is mainly used at the railroad bridge which has strong height limitation because it has greater stiffness than the steel composite girder, or at the continuous girder for building structure because the encasing concrete could resist against compressive stress generated by negative moment.
But, the SRC composite girder is more expensive than the reinforced concrete structure due to filled-in steel beam, and-the structural and economical efficiency are suddenly decreased when the span is longer than 30 m because the dead weight of the structure increase rapidly.
As shown at FIG. 3, the Preflex composite girder (30) has the tension flange which is encased high-strength reinforced concrete, and large prestress is introduced to the encasing concrete of the tension flange. The Preflex composite girder has advantages in that the girder depth could be decreased because the tensile stress by dead and live load is compensated by the introduced prestress, the construction is easy because the weight of girder is light, and the lifting work is very safe because the weight center of the girder lies in bottom flange concrete.
But, the Preflex composite girder has disadvantages in that huge equipment is required to manufacture the Preflex beam, and its construction is more complicated than that of the Steel composite girder or the SRC composite girder, and the economical efficiency is low. Further, Preflex composite girder has structural defects in that the crack of encasing concrete may occur because the prestress introduced to the encasing concrete could be drastically decreased by the creep and shrinkage of concrete, as a result, the encasing concrete is on cracking state under working loads. The amount of the introduced prestress remained in the bottom flange concrete highly depend on the construction schedule. Moreover, if the span length is longer than 50 m, there is a buckling problem of steel beam when introducing preflexing load, and the economical efficiency is remarkably decreased because the amount of steel used and the construction cost for manufacturing the beam itself sharply increases.
As shown at FIG. 4, said PSC composite girder (40) has a structure wherein prestress is introduced to the concrete using high-strength prestressing steel for the purpose of offsetting the tensile stress arose in the cross-section. Said PSC composite girder has advantages in that the price of material is low, the noise is small, and the maintenance cost is low because the main material is concrete, and the displacement is small because the stiffness of the member is great.
But, the PCS composite girder has disadvantages in that its dead weight is heavy, and the construction process is complicated, and the quality control for concrete is difficult. Especially, after applying the dead load and prestressing load, it is most desirable that the distribution of stress induced to the PCS beam is approach the allowable compression stress at the lower chord of the beam and near the allowable tensile stress at the upper chord of the beam, respectively. However, the tensile stress increases rapidly due to it's heavy dead weight as the span length increases, so the more the prestressing should be introduced, and the intensity of the introduced prestress is limited by the geometric properties because the total stress at upper fiber of the cross-section exceeds the allowable tensile stress when the prestress is large. As a result from above facts, sufficient prestress couldn't be introduced to the lower fiber of cross-section, so the beam having large stiffness to resist the tensile stress generated by dead weight of the beam and by live load, namely the high beam is required, however, this causes increase of the dead weight of the beam. By the reasons above-mentioned, the applicable span length of the PSC composite girder is restricted within maximum 40 m based on the simply supported structure system. Further, the PSC composite girder has problem in that huge equipment is required for transportation and construction because lifting of the precast beam using general sized crane is impossible due to the dead weight when the span length is larger than 30 m.
Thus, though there are somewhat differences according to the structural type, the span length of the beam applicable for the conventional composite girders is restricted within maximum 50 m based on the simply supported structure system by the reason of the structural efficiency, the economical efficiency and of the construction efficiency.
Moreover, the beams used for the conventional composite girder is accompanied with many difficulties for manufacturing a certain curved structure of the plane or of the cross-section because all of them are unified solid cross-sectional structure. Of course, it is possible that manufacturing the member having curved structure for the steel beam, but it is not competitive compared with the member having the other structural type because of steep increment of manufacturing cost and abrupt descent of construction efficiency due to it. That is, an expensive steel or concrete box-type girder is more commonly used when the object structure is a curved bridge or curved structure that could not be corresponded with a straight-line type beam than open-type composite girder.